No-three-in-line problem

A set of 20 points in a 10 × 10 grid, with no three points in a line.

In mathematics, in the area of discrete geometry, the no-three-in-line-problem, introduced by Henry Dudeney in 1917, asks for the maximum number of points that can be placed in the n × n grid so that no three points are collinear. This number is at most 2n, since if 2n + 1 points are placed in the grid some row will contain three points.

Lower bounds

Paul Erdős (in Roth, 1951) observed that, when n is a prime number, the set of n grid points (i, i² mod n), for 0 ≤ i < n, contains no three collinear points. When n is not prime, one can perform this construction for a p × p grid contained in the n × n grid, where p is the largest prime that is at most n. As a consequence, for any ε and any sufficiently large n, one can place

(1 − ε)n

points in the n × n grid with no three points collinear.

Erdős' bound has been improved subsequently: Hall et al. (1975) show that, when n/2 is prime, one can obtain a solution with 3(n - 2)/2 points by placing points on the hyperbola xy ≡ k (mod n/2) for a suitable k. Again, for arbitrary n one can perform this construction for a prime near n/2 to obtain a solution with

(3/2 − ε)n

points.

Conjectured upper bounds

Guy and Kelly (1968) conjectured that one cannot do better, for large n, than cn with

$c = \sqrt[3]{\frac{2\pi^2}{3}} \approx 1.874.$

As recently as March, 2004, Gabor Ellmann notes an error in the original paper of Guy and Kelly's heuristic reasoning, which if corrected, results in

$c = \frac{\pi}{\sqrt 3} \approx 1.814.$

Applications

The Heilbronn triangle problem asks for the placement of n points in a unit square that maximizes the area of the smallest triangle formed by three of the points. By applying Erdős' construction of a set of grid points with no three collinear points, one can find a placement in which the smallest triangle has area

$\frac{1-\epsilon}{2n^2}.$

Generalizations

A noncollinear placement of n points can also be interpreted as a graph drawing of the complete graph in such a way that, although edges cross, no edge passes through a vertex. Erdős' construction above can be generalized to show that every n-vertex k-colorable graph has such a drawing in a O(n) x O(k) grid (Wood 2005).

Non-collinear sets of points in the three-dimensional grid were considered by Pór and Wood (2007). They proved that the maximum number of points in the n x n x n grid with no three points collinear is Θ(n²). Similarly to Erdős's 2D construction, this can be accomplished by using points (x, y, x²+y²) mod p, where p is a prime congruent to 3 mod 4. One can also consider graph drawings in the three-dimensional grid. Here the non-collinearity condition means that a vertex should not lie on a non-adjacent edge, but it is normal to work with the stronger requirement that no two edges cross (Pach et al. 1998; Dujmović et al. 2005; Di Giacomo 2005).

Small values of n

For n ≤ 40, it is known that 2n points may be placed with no three in a line. The numbers of solutions (not counting reflections and rotations as distinct) for small n = 2, 3, ..., are

1, 1, 4, 5, 11, 22, 57, 51, 156, 158, 566, 499, 1366, ... (sequence A000769 in OEIS).

References

• Dudeney, Henry (1917). Amusements in Mathematics. Edinburgh: Nelson. 
• Emilio Di Giacomo, Giuseppe Liotta, and Henk Meijer (2005). "Computing Straight-line 3D Grid Drawings of Graphs in Linear Volume". Computational Geometry: Theory and Applications 32 (1): 26–58. doi:10.1016/j.comgeo.2004.11.003. 
• Vida Dujmović, Pat Morin, and David R. Wood (2005). "Layout of Graphs with Bounded Tree-Width". SIAM Journal on Computing 34 (3): 553–579. doi:10.1137/S0097539702416141. 
• Stefan Felsner, Giuseppe Liotta, and Stephen K. Wismath (2003). "Straight-Line Drawings on Restricted Integer Grids in Two and Three Dimensions". Journal of Graph Algorithms and Applications 7 (4): 363–398. http://jgaa.info/accepted/2003/Felsner+2003.7.4.pdf. 
• Flammenkamp, Achim (1992). "Progress in the no-three-in-line problem". Journal of Combinatorial Theory. Series A 60 (2): 305–311. doi:10.1016/0097-3165(92)90012-J. 
• Flammenkamp, Achim (1998). "Progress in the no-three-in-line problem, II". Journal of Combinatorial Theory. Series A 81 (1): 108–113. doi:10.1006/jcta.1997.2829. 
• Guy, R. K.; Kelly, P. A. (1968). "The no-three-in-line problem". Canadian Mathematical Bulletin 11 (0): 527–531. doi:10.4153/CMB-1968-062-3. MR0238765. 
• Hall, R. R.; Jackson, T. H.; Sudbery, A.; Wild, K. (1975). "Some advances in the no-three-in-line problem". Journal of Combinatorial Theory. Series A 18 (3): 336–341. doi:10.1016/0097-3165(75)90043-6. 
• Lefmann, Hanno (2008). "No l Grid-Points in spaces of small dimension". Algorithmic Aspects in Information and Management, 4th International Conference, AAIM 2008, Shanghai, China, June 23-25, 2008, Proceedings. Lecture Notes in Computer Science. 5034. Springer-Verlag. pp. 259–270. doi:10.1007/978-3-540-68880-8_25. 
• Pach, János; Thiele, Torsten; Tóth, Géza (1998). "Three-dimensional grid drawings of graphs". Graph Drawing, 5th Int. Symp., GD '97. Lecture Notes in Computer Science. 1353. Springer-Verlag. pp. 47–51. doi:10.1007/3-540-63938-1_49. 
• Attila Pór and David R. Wood (2007). "No-three-in-line-in-3D". Algorithmica 47 (4): 481. doi:10.1007/s00453-006-0158-9. 
• Roth, K. F. (1951). "On a problem of Heilbronn". Journal of the London Mathematical Society 26 (3): 198–204. doi:10.1112/jlms/s1-26.3.198. 
• David R. Wood (2005). "Grid drawings of k-colourable graphs". Computational Geometry: Theory and Applications 30 (1): 25–28. doi:10.1016/j.comgeo.2004.06.001. 

External links

• Flammenkamp, Achim. "The No-Three-in-Line Problem". http://wwwhomes.uni-bielefeld.de/achim/no3in/readme.html. 

• Weisstein, Eric W., "No-Three-in-a-Line-Problem" from MathWorld.